Circuit arrangements of the above mentioned general type are used, for example, in transponders for wireless or contactless identification systems such as RFID systems (radio frequency identification systems) and in remote sensors. In this context, the power derived from the electromagnetic field operates the transponder or remote sensor in order to transmit, or especially backscatter, identification information or sensor information in a contactless manner from the transponder or sensor to the base station or reader station. Throughout this specification, the general term “transponder” should be understood to cover both passive and semi-passive transponders, as well as remote sensors that include sensor elements in or connected to a transponder. The term “receiving/backscattering device or arrangement” refers to any device that receives and backscatters an electromagnetic wave signal, and includes transponders, remote sensors, and the like.
Such transponders, or particularly the receiving/backscattering or receiving/transmitting circuit arrangements thereof, typically do not include an active transmitter for actively transmitting the relevant data to the base station. Instead, the transponders are non-active systems that can be designated as passive systems if they do not comprise their own power supply, or as semi-passive systems if they do comprise their own power supply. In such non-active systems, the data transmission in the distant field of the base station in connection with UHF waves or microwaves generally uses a so-called backscattering or backscatter-coupling between the receiving/backscattering arrangement (i.e. transponder) and the base station. While such non-active transponders are sometimes also called receiver/transmitters or receiving/transmitting arrangements, this application uses the term “receiving/backscattering arrangement”, because this arrangement does not actively transmit but rather merely reflects or backscatters the modulated received electromagnetic waves.
In this regard, the base station emits electromagnetic waves, which are received by the transponder, and then modulated by the transponder in accordance with the particular data that are to be transmitted back to the base station. The thusly modulated signal or electromagnetic waves are reflected or backscattered with the modulation back to the base station. This modulated reflection or backscattering is generally achieved using an input impedance variation of the receiving/backscattering arrangement, which causes a corresponding variation of the reflection characteristics of an antenna connected thereto. Thereby the reflected or backscattered signal is correspondingly modulated, so that the base station can receive and evaluate the modulated backscattered signal so as to recover the data represented by the modulation.
The demands on the transmission range of transponders and particularly passive transponders are continuously increasing in various technical fields or applications. In order to achieve relatively large transmission ranges of passive transponders, the antenna or the circuitry connected to the antenna of the transponder must, respectively, be suitably designed and particularly dimensioned in a matched manner. Also, a rectifier included in the transponder, which receives and rectifies the alternating voltage signal that was received by the antenna to provide the operating power, must be able to convert even the smallest voltages of the input signal into a sufficient operating voltage at the output side of the rectifier. The corresponding high sensitivity of the rectifier that is required in this regard may, however, lead to destruction of the rectifier itself or other circuit components when the transponder operates in the near field or close range of the base station, due to the higher field strength of the emitted electromagnetic field prevailing there. Namely, the higher received field strength may, for example, cause the rectifier to generate an output voltage that is too high, so that it must necessarily be limited by a voltage regulator. Depending on the particular design or embodiment of the voltage regulator, this can lead to an increased current flow in the rectifier, which may cause the generated or dissipated power to exceed the maximum permissible power and thereby may destroy the rectifier or other circuit components.
In order to limit the power that is received and extracted from the electromagnetic field via the antenna into the circuit arrangement, particularly in the near field or close range of the emitting base station where the field strength is high, a so-called detuning of the input circuit connected to the antenna may be carried out. This purposeful impedance mis-matching of the input circuit causes an increase of the reflected power proportion and a corresponding decrease of the absorbed power proportion.
In conventional systems, such detuning is carried out by means of a detuning unit that uses a modulator performing an amplitude shift keying (ASK) in order to vary the real part of the input impedance of the input circuit or circuit portion connected to the antenna so as to detune or mis-match the input impedance. For this purpose, the ASK modulator is arranged in the circuit input portion or as an additional load resistor at the output of the rectifier. Such a circuit arrangement is known, for example as described in the published European Patent Application EP 1 211 635 A2.
Such a known detuning by means of ASK modulation, however, requires a high or large real part of the input impedance in comparison to the imaginary part thereof. In this context, the ASK modulator is controlled or actuated not only by the ASK modulation control signal but also by a regulator arranged in the detuning unit, which achieves a suitable mis-matching or detuning through an operating point adjustment of the ASK modulator dependent on the power being supplied or fed-in through the antenna.
In view of the above, problems or difficulties arise if such a detuning is to be used in connection with a phase shift keying (PSK) modulation for carrying out the data transmission between the transponder and the base station. Namely, for such a PSK modulation, a smaller real part in comparison to the imaginary part is required for the input impedance. Thus, in order to adjust the detuning or mis-match by varying the real part, it becomes necessary to provide a low resistance path in the input portion of the circuit or at the output of the rectifier through an arrangement of an additional switching device, for example a transistor. Such low resistance switching devices generally comprise relatively large parasitics, which lead to corresponding losses in normal operation, whereby the transmission range of the transponder is correspondingly reduced. For this reason, it is generally not possible to combine a PSK modulator for carrying out a PSK modulation for the data transmission, with a detuning unit based on varying the real part of the input impedance, i.e. based on an ASK modulation.
From the German Patent Laying-Open Publication DE 196 29 291 A1, it is known to arrange two anti-parallel diodes in an input circuit of a transponder for providing electrostatic discharge (ESD) protection. In this context, the diodes become conductive when, for example due to an ESD interference, an unusually high and interfering input voltage exceeds a diode threshold voltage of the diodes. A field strength dependent power adaptation is not provided with such an ESD protection circuit.